This invention relates generally to an improved piezoelectric crystal oscillator circuit for use in an electronic timepiece. It is particularly directed to the provision of a tuning fork quartz crystal oscillator which maintains frequency stability during variations in the supply voltage.
Recently, watches have been developed which utilize quartz crystal vibrators to provide a high frequency time standard signal.
The application of this type of oscillator circuit to timekeeping, particularly in wristwatches, imposes a number of conditions upon the design of the oscillator circuit, VIZ:
it must have a very low power consumption, PA1 It must be capable of operating from a very low voltage supply, PA1 It must sustain oscillation during temporary, but substantial, supply voltage drops caused, for example, during display illumination/actuation, PA1 It must maintain high frequency stability with supply voltage decreases during the life of the battery, and PA1 The crystal vibrator must be inexpensive.
Some known embodiments of crystal-controlled oscillators which utilize crystal vibrators/resonators that are practically priced for use in wristwatches comprise tuning fork type vibrators across the terminals of an amplifier of integrated circuit design. As seen from the prior art, however, the operation of this kind of crystal device in heretofore known oscillator/amplifier wristwatch circuits gives rise in particular to such loop phase conditions and circuit limitations/constraints which result in inefficient crystal operation, i.e., at its anti-resonant frequency characteristic, and therefore, necessitated the use of a relatively high voltage/current supply source. The result, especially where application involves wristwatch circuits supplied by generally available button-type wristwatch batteries whose output voltage decreases progressively during their operation, is that the time of utilization of these batteries is reduced to that period of their operation in which the voltage level is sufficient to maintain the precise loop phase conditions and also typically necessitated the use of more than one such battery to provide the required voltage/current needs of the oscillator to sustain divider or counter circuit actuation, particularly during display illumination/actuation.
The prior art includes a number of crystal oscillator circuits such as is described in U.S. Pat. No. 3,935,546, issued Jan. 27, 1976 to Shinji Morozumi et al., U.S. Pat. No. 3,824,495, issued July 16, 1974 to Erich Gerum, U.S. Pat. No. 2,946,018, issued July 19, 1960 to S. K. Benjamin et al., U.S. Pat. No. 3,911,378, issued Oct. 7, 1975 to James E. Buchanan and U.S. Pat. No. 3,761,840 issued Sept. 25, 1973 to Gordon F. Bremer.
In contrast with the prior art, the present invention provides an improved crystal oscillator circuit, particularly an oscillator circuit which utilizes a three terminal tuning fork type crystal vibrator, which satisfies the above noted conditions, utilizes an integrated circuit amplifier to provide both regenerative feedback and self bias stabilization, operates the crystal vibrator substantially at its peak efficiency, reduces oscillator voltage/current level requirements, prolongs the useful life of the voltage/current source and/or enables practical use of a single button-type wristwatch battery.